1. Field of the Invention
The present invention described herein relates to the field of preparing preforms which are used to make optical fibers. More specifically, the invention relates to making preforms for halide glass fibers. The invention especially relates to making preforms for fluoride glass fibers.
2. Description of the Prior Art
Simple chemical vapor deposition processes are well known in the manufacture of silica fibers. More specifically, simple chemical vapor deposition process are well known in the manufacture of fluoride glass fibers for low attenuation and good transmission beyond 2 microns, that is, in the infrared range. In such processes, organometallic precursors of BeF.sub.2 and AlF.sub.3 are volatilized and carried by an inert gas into a silicate glass tube. There the precursors break down, yielding the fluoride products which deposit on the inner wall of the tube, while the remaining products flow out the exhaust. The fluoride materials are sintered by means of a travelling heat coil into a BeF.sub.2 glass layer (forming the cladding of the preform), and a BeF.sub.2 /AlF.sub.3 glass layer of slightly higher refractive index (forming the core of the preform). This preform is then drawn into an optical fiber capable of infrared transmission.
Although the simple chemical vapor deposition process yields preforms of proper stoichiometry and reasonable purity, there are a number of limitations in its use for fluoride or other halide materials due to the intrinsic characteristics of halides.
First, halide material precursors have low vapor pressures. Preform fabrication by simple chemical vapor deposition at the usual atmospheric pressures would be a slow process.
Second, halide materials generally have low softening temperatures. Once chemical vapor deposition occurs, often the material is in the form of soot or powder, and the tube must be heated to above the softening temperature so as to change the products from soot to a dense glass film. Since the deposition tube is also likely to be made of low melting halide glass, it may soften and lose its shape during this process.
Third, most halide glasses consist of several components. With simple chemical vapor deposition methods, after each component precursor is carried by inert gas to the deposition tube, conditions for deposition must be chosen such that the resulting deposited layer contains the proper mixture to form the desired halide composition. Such a process would certainly be difficult for a typical six-component halide glass composition.
Fourth, simple chemical vapor deposition processes are generally slow and inefficient processes. The generally poor efficiency of simple chemical vapor deposition processes would be further worsened due to inherently low deposition rates with halide materials.
Other processes are known for producing internally coated glass tubes for drawing of fiber optic light conductors. For example, in U.S. Pat. No. 4,145,456 of Kuppers et al a method of producing internally coated glass tubes for the drawing of fiber optic light conductors is disclosed. In the method, a plasma is created to coat the inside of a glass tube. The plasma is created in the presence of SiCl.sub.4, oxygen, and GeCl.sub.4. The coating consists of a plurality of layers of SiO.sub.2 doped with an increasing content of GeO.sub.2. In spite of the use of halogen-containing materials such as SiCl.sub.4 and GeCl.sub.4, a suitable halide glass is not disclosed as being obtained. Instead, the coating consists of a plurality of layers of SiO.sub.2 doped with an increasing content of GeO.sub.2. Moreover, there is no disclosure of producing a fluoride glass in the Kuppers et al patent.
In U.S. Pat. No. 4,718,929 of Power et al there is a disclosure of a vapor phase method for making a metal halide material useful for the drawing of an optical waveguide fiber. In the method, a fluorinated beta-diketonate (which is a halogenated organometallic material), e.g. Zr(hfa).sub.4 (column 3, line 47), is treated with an energy source to bring about an intramolecular fluorine transfer reaction through which metal fluorides are formed. The fluorinated beta-diketonate constitutes the principal source of both metal and halogen in the metal halide. The source of energy may be a plasma, e.g. plasma discharges at radio frequencies or microwave frequencies (column 4, lines 39-47).
It is noted that in the Power et al patent, no disclosure is provided for formation of a metal halide via in situ formation of halide from a halide-carrying gas. Furthermore, no disclosure is provided of direct formation of metal halide by exposure of fluorinated alkoxides to a plasma-generating environment.